Downloaded from https://doi.org/10.1079/BJN19950090 British Journal of Nutrition (1995), 73, 851-862 851 https://www.cambridge.org/core Effect of the lipase inhibitor orlistat and of dietary lipid on the absorption of radiolabelled triolein, tri-y-linolenin and tripalmitin in mice BY DOROTHEA ISLER, CHRISTINE MOEGLEN, NIGEL GAINS AND MARCEL K. MEIER . IP address: Pharma Division, Preclinical Research, F. Hoffmann-La Roche Ltd, CH-4002 Basel, Switzerland (Received 8 November 1993 - Revised 12 September 1994 - Accepted 7 October 1994) 170.106.40.139 Orlistat, a selective inhibitor of gastrointestinal lipases, was used to investigate triacylglycerol absorption. Using mice and a variety of emulsified dietary lipids we found that the absorption of , on radiolabelled tripalmitin (containing the fatty acid 16: 0), but not of triolein (18 :ln-9) or tri-y-linolenin 27 Sep 2021 at 17:57:17 (18:3n-6), was incomplete from meals rich in esterified palmitate. Further, the absorption of radiolabelled triq-linolenin, from both saturated and unsaturated dietary triacylglycerols, was 1.3- to 2 fold more potently inhibited by orlistat than that of triolein and tripalmitin. These radiolabelled triacylglycerols, which have the same fatty acid in all three positions, may not always be accurate markers of the absorption of dietary triacylglycerols. Orlistat was more effective at inhibiting the absorption of radiolabelled triacylglycerols with which it was codissolved than those added separately, , subject to the Cambridge Core terms of use, available at which indicates that equilibration between lipid phases in the stomach may not always be complete. The saturation of the dietary lipid had little or no effect on the potency of orlistat. Orlistat provides a novel approach for studying the role of triacylglycerol hydrolysis in the overall process of triacylglycerol absorption. Lipase inhibitor: Triacylglycerol: Absorption: Mice Dietary triacylglycerols can only be absorbed after they have been hydrolysed by gastrointestinal lipases to give fatty acids and monoacylglycerols (Nelson & Ackman, 1988). In vitro, different triacylglycerols are hydrolysed at different rates; for example, triacylglycerols that contain less common or long-chain, saturated fatty acids are hydrolysed more slowly (Brockerhoff, 1970; Lawson & Hughes, 1988; Bergstedt et QZ. 1990, 1991; Yang et al. 1990). In vivo the rate of fatty acid absorption depends on the physicochemical properties both of the individual fatty acids and of the dispersion that they https://www.cambridge.org/core/terms form with the fats and other components in the diet (Ockner et al. 1972; Carey et al. 1983). The site of absorption has also been found to depend on the rate of absorption; triacylglycerols that are more slowly absorbed, for example those with long-chain, saturated fatty acids, tend to have a more distal site of absorption (Bergstedt et al. 1990, 1991). However, the large excess of both pancreatic (EC 3.1.1.3) and carboxyl ester lipase (EC 3.1.1.13) combined with the large surface area of the intestine ensure that even alkyl esters with low rates of hydrolysis are efficiently absorbed (Nelson & Ackman, 1988; Chen et al. 1989; Yang et al. 1990). Orlistat is a selective and potent inhibitor of lipases that acts solely in the intestinal lumen . and does not inhibit the absorption of oleic acid (Borgstrom, 1988; Hadvary et aZ. 1988, Address for correspondence: Dr D. Isler, F. Hoffmann-La Roche Ltd, Bau 60/403, Grenzacherstr. 124, CH-4002 Basel, Switzerland. Downloaded from https://doi.org/10.1079/BJN19950090 852 D. ISLER AND OTHERS 1991; Fernandez & Borgstrom, 1989; Luthi-Peng et al. 1992). It inactivates the lipase by reacting covalently with the serine (Se~-l~~)in the active site. Orlistat has been shown to https://www.cambridge.org/core reduce hypertriacylglycerolaemia and obesity in animals (Hogan et al. 1987; Meier et al. 1991) and its effectiveness in man is undergoing clinical investigation (Drent et al. 1992; Hauptmann et al. 1992). Our aim was to use orlistat in order to study the relative efficiency with which a variety of emulsified dietary lipids are absorbed in mice, and in particular to answer the following questions : fmt, whether radiolabelled triacylglycerols, such as triolein (containing the fatty acid 18 : ln-9), tri-y-linolenin (18 :3n-6) and tripalmitin (1 6 :0), are suitable markers for the absorption of dietary triacylglycerols; and second, under conditions where triacylglycerol . IP address: hydrolysis is impaired, whether the absorption of these radiolabels is affected by the nature of the dietary fat in which they are dissolved. 170.106.40.139 MATERIALS AND METHODS Materials , on [14C]Triolein(glycerol-tri[ l-14C]oleate, 2 GBq/mmol), [14C]tripalmitin(glycerol-tri[ 1-14C]- 27 Sep 2021 at 17:57:17 palmitate, 2 GBq/mmol), [ l-14C]oleicacid (2 GBq/mmol) and [14C]dioleoyl-phosphatidyl- choline (1,2-di[ 1-'4C]oleyl-3-phosphatidylcholine, 4 GBq/mmol) were obtained from Amersham International plc (CH-8058, Zurich, Switzerland),[3H]triolein (glycerol-tri[9,10- 3H]oleate, 992 GBqjmmol) from NEN Research Products Du Pont de Nemours International S.A., CH-8058 Zurich, Switzerland, and [14C]tri-y-linolenin(glycerol-tri- [1-14C]-6,9,12-~~tadecatrienoate,7 GBqlmmol) was prepared by Dr H. Harder at , subject to the Cambridge Core terms of use, available at F. Hoffmann-La Roche, Basel, Switzerland. Briefly, [14C]tri-y-linolenicacid was prepared by treating l-chloro-5,8,1l-heptadecatrienewith Rieke's magnesium (Rieke & Bales, 1974) and allowing the resulting Grignard intermediate to absorb 14C0,; [14C]tri-y-linoleninwas prepared by reacting the [14C]tri-y-linolenic acid with glycerol in the presence of dimethylaminopyridineand dicyclohexylcarbodiirnide. The [14C]tri-y-linoleninwas stored, in portions sufficient for one experiment, in sealed ampoules under Ar. Radioactive purity was checked by TLC using precoated silica-gel plates (HPTLC 60; Merck (Schweiz) AG, CH-8029 Zurich, Switzerland) and di-isopropylether-heptane-acetic acid (48 : 32 : 1.6) for triacylglycerols, chloroforn-methanol-acetic acid-water (65 : 50 :4 : 1) for phosphatidylcholine, and heptane-isopropyl ether-acetic acid (60 :40 :4) for oleic acid. Radioactive purity was at least 97 'YO. [1,2-3H]polyethyleneglycol4000 was from NEN Research Products. Linseed oil was from a local pharmacy, olive and sunflower oils, cream and skimmed milk powder were from a supermarket, palm and coconut oils were from Tensochema AG, https://www.cambridge.org/core/terms Zurich, Switzerland, borage oil was from Roche Lipid Technology, Heanor, Derbyshire and egg phosphatidylcholine was from Lipid Products, South Nutfield, Surrey. Orlistat (Ro 18-0647/002, MW 496) was obtained by chemical synthesis (Barbier & Schneider, 1987). All other reagents were of analytical grade or the purest available. Animals Female, MORO (Ibm), albino mice (about 25 g; twenty to thirty mice per experiment) were maintained on chow (Kliba 25-331 ; Klibamuhle, Kaiseraugst, Switzerland) and water ad lib. They were kept individually in wire-mesh cages under a 12 h light-12 h dark cycle. Preparation of the test meals The radiolabelled triacylglycerols, with or without orlistat, were dissolved in the cream and in the plant oils, under a stream of N,, by stirring for about 30 min at room temperature (at 65" for [14C]tripalmitin)to give about 0.037 MBq/g plant oil and cream fat. Downloaded from https://doi.org/10.1079/BJN19950090 ORLISTAT AND TRIACYLGLYCEROL ABSORPTION 853 Table 1. Approximate fatty acid composition (g/100 g total fatty acids), iodine values and https://www.cambridge.org/core melting points (") of the dietary fats Palm Coconut Olive Sunflower- Borage Linseed oil*? oil? Cream*? oil* seed oil* oils oil* ~ Polyunsaturated Linoleic 10 2 2 8 69 36 15 Linolenic 1 1 2 1 0 23 53 Monounsaturated . IP address: Oleic 37 7 23 74 19 16 21 Others 0 0 7 3 1 8 0 Saturated < Cl? 2 80 21 0 tl 0 <1 170.106.40.139 Palmitic 45 8 45 12 7 12 6 'c,, 3 2 0 2 4 5 5 Iodine value 53 9 34 90 131 166 210 Melting points 40 27 -4 - 25 -28 -30 , on * Wissenschaftliche Tabellen Geigy (1977). 27 Sep 2021 at 17:57:17 t According to the suppliers. $ Raederstorff & Moser (1992). 8 For details of procedure, see p. 854. The liquid test meals contained (g/kg) either starch 25, glucose 240, defatted milk powder 120, plant oil 76 and 150 mM-NaC1 solution 539, or, in order to obtain an , subject to the Cambridge Core terms of use, available at equivalent composition, starch 18, glucose 173, defatted milk powder 87, cream 333 (i.e. 76 g fat/kg) and NaCl solution 389. The fatty acid compositions of the dietary fats are given in Table 1. A stable emulsion was made by suspending the milk powder, glucose and starch in the NaCl solution, adding the dietary fat and blending in a glass-on-glass homogenizer for 30 s (2 min for cream). As described below, these test meals contained one, or more usually, two radiolabelled triacylglycerols. After preparation the test meals were stored at -20". On the day of use they were thawed and reblended (30s). On both the day of preparation and of use the stability of the emulsions and the uniform distribution of the markers were checked by measuring the radioactivity of at least five samples from different sites within the emulsion. Administration of test meals and orlistat suspensions The vehicle in which orlistat was dispersed contained 50 g gum arabic plus 50 g milk powder/l in water. https://www.cambridge.org/core/terms The mice were divided into groups of three. After a 24 h fast, each mouse was orally administered about 250 pl(l0 ml/kg body weight) of the liquid test meal followed directly by about 250 ,ul of the above vehicle.